1. Field of the Invention
The present invention relates to a constant-current polarization voltage detecting method used suitably in a Karl Fischer's moisture content measuring method and a Karl Fischer's moisture content measuring apparatus using such detecting method.
2. Discussion of the Background
For a detecting means to measure a moisture content for which a Karl Fischer's (hereinbelow, referred to as "KF") volumetric titration method of this kind is used, a so-called constant-current polarization voltage detecting method has often been employed in recent years. Namely, in the constant-current polarization voltage detecting method, a minute current is fed to dual platinum electrodes, as a detecting electrode, to measure a voltage across the dual platinum electrodes. In this case, although a minute current to be fed can either be a direct current or an alternating current, a pulse-like current has recently been used.
On the other hand, when a titration solvent composed mainly of chloroform as solvent was used for the constant-current polarization voltage detecting method, there was in fact difficulty in conducting the measurement of a large amount of sample in a moisture content measuring apparatus by a conventional technique. This was because the monitoring of a polarization voltage could not properly be effected since the liquid resistance of a chloroform solvent is high, and the liquid resistance is further increased by the incorporation of a large amount of sample having no-polarization properties in the titration solvent.
The moisture content measurement using the above-mentioned KF volumetric titration method is such one utilizing KF titration reactions as follows: EQU I.sub.2 +SO.sub.2 +H.sub.2 O+3BASE.fwdarw.2BASE.HI+BASE.SO.sub.3 EQU BASE.SO.sub.3 +CH.sub.3 OH.fwdarw.BASE.HSO.sub.4 CH.sub.4
wherein BASE: an amine compound
Namely, in the titration reactions in the moisture content measurement by the KF volumetric titration method, since the reaction with water accelerates selectively, a wide application has conventionally been made for the measurement of moisture content. In this case, in a KF volumetric titration method, measurement is carried out by using an iodine-containing solution as titrant. In a coulometric titration method, iodine is generated by anode oxidation of iodide ions. Thus, the above-mentioned reactions are effected. In either method of detection, the end point of titration is recognized when an excessive amount of iodine is detected on the detecting electrode.
In such detecting method for detecting an excessive iodine, the above-mentioned constant-current polarization voltage detecting method is generally used, wherein according to the volumetric titration method, the titration is finished at the time when a state that an amount of iodine is excessive (a state a potential which is lower than that at the end point) continues 30 seconds, and according to the coulometric titration method, the titration is finished at the time when a potential is beyond the potential at the end point at which a state of a slight iodine being excessive can be detected.
Generally, there is no problem in a conventional titration method even when solvent containing much methanol is used as a titration solvent, and the titration is conducted by using a KF reagent. Even though any sample is incorporated, it is possible to reach the end point if the KF reagent is dropped excessively, and the moisture content measurement can be conducted regularly.
However, in a case of measuring oils, the titration is usually conducted in a titration solvent containing chloroform as a main solvent because the oils do not dissolve in methanol. However, since oils generally have a small moisture content, a large amount of sample is to be poured into the titration solvent. In this case, the presence of a much amount of chloroform increases the liquid resistance of the titration solvent. The incorporation of a much amount of sample further increases the liquid resistance. Under such condition, when a predetermined minute current of pulse form is applied, an apparent polarization voltage is assumed to be the sum of the true polarization voltage and a polarization voltage resulted from the liquid resistance. Accordingly, even though the titration in fact reaches the end point of titration, there is found a phenomenon as if it does not reach the end point of titration.